Selenium is an essential trace element currently under consideration as a chemopreventive agent. Although there is over 25 years of animal studies, and human clinical data as well, indicating the efficacy of selenium in cancer prevention, the mechanism of action remains unknown. The long term goals of his proposal are to determine the mechanism by which selenium prevents cancer and gain a better understanding of how selenium regulates the synthesis of selenium- containing proteins. In the first Specific Aim, the hypothesis that selenium reduces the levels of DNA damage following radiation exposure will be investigated by providing selenium, and the glutathione peroxidase mimetic drug ebselen, following irradiation and quantifying oxidative lesions in DNA and DNA strand breaks. Changes in the expression of stress response genes due to low level selenium supplementation of tissue culture media will also be investigated. In the second Specific Aim, the mechanism by which selenium regulates selenoprotein biosynthesis will be determined by focusing on selenium- induced methylation of the dedicated tRNA molecule that recognizes in- frame UGA codons within selenoprotein mRNAs as the amino acid selenocysteine. The role of SECIS (Selenocysteine insertion sequence) elements present in the 3' untranslated region of selenoprotein mRNAs in the translational regulation of selenoprotein biosynthesis will also be assessed. This will be accomplished using a specialized reporter construct that permits the rapid quantification of UGA readthrough under a variety of experimental conditions. In the third Specific Aim, a transgenic mouse model in which selenoprotein levels are reduced due to the presence of a mutated selenocysteine tRNA, will be used to establish the role of selenoproteins in the protection offered by selenium supplementation. Collectively, these studies will provide new insight on how selenium protects cells from radiation-induced mutagenesis and may lead to novel approaches to increase the beneficial properties of this nutrient.